Organic light-emitting devices that operate by electroluminescence (hereinafter abbreviated as EL) from organic materials, (what are called organic EL devices), are completely solid devices of a thin film type capable of emitting light at a low voltage ranging from several V to several 10 V and have many good characteristics such as high luminance, high luminous efficiency, small thickness, and lightweight. In recent years, therefore, they have attracted attention as backlights for a variety of displays, display boards for signboards, emergency lights, and the like, and surface emitting devices for illumination light sources and the like.
Such organic light-emitting devices have a structure including two electrodes and a light-emitting layer including an organic material disposed between the electrodes, in which light produced by the light-emitting layer is extracted outside through the electrode. Therefore, at least one of the two electrodes is formed as a transparent electrode.
Transparent electrodes generally used include oxide semiconductor materials such as indium tin oxide (SnO2—In2O3, ITO). A stack of ITO and silver layers is also studied to provide lower resistance (e.g., refer to Patent Literatures 1 and 2). Unfortunately, ITO, which contains a rare metal, indium, is a high-cost material and needs to be annealed at about 300° C. for resistance reduction after it is deposited as a film. In addition, when ITO is deposited on a film substrate, a decrease in deposition temperature can occur to degrade the crystallinity, so that a problem can occur, such as failing to obtain an appropriate electrode resistance.
In contrast, a silver electrode film with very high conductivity can be formed even on a film substrate. Although has been previously studied widely, such a silver electrode trades off transparency for conductivity, and no transparent electrode has been practically achieved using a thin silver film.
To solve this problem, Patent Literature 3 shows the use of a silver-aluminum alloy. The alloy has improved transparency and conductivity as compared with a conventional one. Unfortunately, such improved transparency and conductivity are still at an unsatisfactory level.
On the other hand, Japanese Patent Application 2011-252003 by the present applicant discloses a thin silver transparent electrode that has significantly improved transparency and conductivity as compared with a conventional one. Such a thin silver transparent electrode is a very preferred mode.
However, a thin silver electrode has a problem with light distribution characteristics due to the effect of its unique optical properties. When a thin silver electrode is used, a method of simply providing a light extraction film or the like on the outside of the substrate (the interface between the substrate and the air) has limitations in improving luminous efficiency.